The invention was designed as part of a personal communicator that combines a cellular phone, a data and fax modem, and a data processing system, into a mobile, compact, lightweight, battery operated, handheld device having a variety of data processing and communication functions. These functions include operating the device as a standard cellular phone; a calendar showing a list of things to do at various times; an address book for organizing names, addresses and phone numbers; a calculator; a note pad allowing text entry and editing; and a communication device for fax, E-mail, and various data bases and services.
The personal communicator is in the form of a handset having an earphone or speaker at one end, a microphone at the other end, and a small touchscreen display between the speaker and the microphone. The display has a touch or pressure sensitive overlay, and a fine pitch liquid crystal display (LCD) operated as a standard CGA display with a resolution of 640.times.200 pixels. The touchscreen display provides a graphic user interface, input/output device for a variety of functions or applications.
The general problem, which the invention addresses, is that due to the mobility of the communicator, the LCD may be exposed to wide temperature extremes which affect the LCD contrast. Extreme cold renders the liquid crystal material opaque and extreme heat makes the material transparent. Thus, some form of contrast control is needed to allow the user to adjust the contrast of the LCD to compensate for the extreme temperature effects. Such a control should also be usable to allow a user to adjust contrast to suit personal preferences.
This general problem is known in the art, but the solutions differ as to how contrast control is provided. U. S. Pat. No. 5,029,982-Nash discloses an "LCD CONTRAST ADJUSTMENT SYSTEM" for a portable computer. The system includes a user adjustable power supply which the user sets to achieve a desired contrast. A temperature sensor is used to generate a correction voltage that is added to the power supply voltage to maintain the user setting. The patent is silent as to the apparatus for adjusting the power supply. Presumably, adjustment is done by a standard knob or external, manually actuated slider or rotary knob.
The problem becomes more complex due to a design restraint of limiting the external controls to the three buttons discussed above and not allowing any additional external button or knob that would be dedicated to contrast control. Furthermore, the use of the touchscreen display itself to provide the contrast adjustment, appears to be ruled out since any simulated control buttons may not be visible under the extreme temperature conditions.